1. Field of the Invention
This invention relates to the drilling and workover of wells penetrating subterranean formations, particularly high-temperature formations, such as geothermal formations, and more particularly to the drilling of such formations with foam drilling fluids.
2. Description of the Prior Art
It has long been conventional practice in the drilling of bore holes penetrating subterranean formations to circulate a drilling fluid down the drill pipe and back up the bore hole annulus for the removal of debris and drill cuttings from the bore hole. Drilling fluid also serves to prevent sloughing in the bore hole during the drilling operation. Recently, air and gas have replaced conventional drilling fluids in the drilling of some bore holes, and have proved particularly beneficial in formations wherein dense conventional drilling fluids would be lost to the formation. Additionally, air and gas have also provided longer bit life and higher rates of penetration in the drilling of bore holes.
However, in many formations, water enters the bore hole from adjacent water-containing strata; and air and gas have proved unable to remove this water. In view of this problem, foams have been developed to remove both the cuttings and the water from these bore holes. Numerous foam compositions have been proposed and used; however, as yet, foams have not been satisfactorily used in the drilling and workover of wells penetrating high-temperature subterranean formations, such as geothermal formations.
At present, geothermal formations are drilled using air or other gases as the drilling or workover fluid. This is particularly true in the vapor-dominated geothermal reservoirs wherein the low bottom-hole pressure precludes the use of heavy drilling fluids, such as conventional drilling muds. If used, circulation of these conventional drilling fluids would quickly be lost to the low-pressure formation. Also, the high temperatures of the geothermal reservoirs have precluded the use of foams normally used in the typically low-temperature, oil-bearing formations. While the temperature of typical oil-bearing formations amenable to foam drilling may reach as high as 200.degree. F., the geothermal wells now being drilled have bottom-hole temperatures of from about 400.degree. F. to about 700.degree. F. Because these high-temperatures greatly reduce foam stability, foams have not been used as drilling or workover fluids in wells penetrating high-temperature subterranean formations, such as wells penetrating geothermal formations. Thus there is a need for a drilling and workover method which employs a foam that is stable at high temperatures and which can be used in high-temperature subterranean formations.
Accordingly, a principal object of this invention is to provide a method of conducting foam drilling and workover operations in subterranean formations.
Yet another object of this invention is to provide a method of conducting foam drilling and workover operations in high-temperature subterranean formations.
Still another object of this invention is to provide a method of conducting foam drilling and workover operations in high-temperature geothermal formations.
A still further object of this invention is to provide a method of conducting foam drilling and workover operations in high-temperature subterranean formations which employs a foaming agent that is stable at the temperatures encountered.
Yet still another object of this invention is to provide a method of conducting foam drilling and workover operations in geothermal formations which employs a foaming agent that is stable at the temperatures encountered.
Other objects and advantages of this invention will become apparent to those skilled in the art from the following description.